Vol. 55.] DK. A. W. EOWE ON THE GENUS MICKASTEE. 541 



This position is not assumed in a carping spirit, or on theoretical 

 grounds, but it is the outcome of personal and rigid zonal collecting, 

 and of the critical examination of a large series of specimens as soon 

 as they are removed from the cliff or quarry. 



We must either make a species out of every trivial variation, or 

 mass certain obviously allied forms into groups. The latter plan 

 seems singularly well adapted to the genus in question, as it gives 

 us an opportunity of handling a prolific and variable series, such as 

 that of M. prcecursor, in a broad and effective manner. It is 

 only by this means that passage-forms and mutations can be 

 intelligibly arranged ; and as they form the bulk of the genus, it 

 seems unnecessary to offer further excuse for the course adopted. 

 Unless we unite these variations into a group which will admit 

 of the zoological continuity being exemplified, and the zonal 

 peculiarities noted, we must make a species for each variation, and 

 that in each zone. The one plan is sufficient for all purposes of 

 nomenclature, and at the same time it marks the evolution of the 

 genus, zone by zone ; the other is mere ticketing, and meaningless 

 ticketing to boot ! Judged by this standard, it would seem impossible 

 to take a Micraster from a chalk-pit, and to give it a new specific 

 name, simply because the curve of the superior surface, or the angle 

 of posterior truncation, differed slightly, or even considerably, from 

 figures already published. If the essential features of the test mean 

 anything, it would only be possible to found a new species when 

 the Micraster differed in its essential features from all other forms in 

 that particular zone. This is the logical outcome of such an inquiry 

 as this, and by it the writer is prepared to stand. 



Great care has been exercised in classifying the essential details 

 of the test in their relationship to the various zones, and the writer 

 believes that these anatomical details are substantially accurate, and 

 free from speculative taint ; moreover, the collecting has been ex- 

 tended over a wide area, so as to avoid local peculiarities. Even if 

 the deductions made in this paper may be proved to be invalid, it 

 is reasonable to think that the method of examining a genus, by 

 taking a large series of zonally-collected specimens in review, might 

 with advantage be followed in the case of other genera, and in 

 other formations. It would tend to bring out the working of 

 evolution, and to reduce the multiplication of so-called species. 



Though it is quite possible that a Micraster may be picked out 

 which may not have all the essential features herein demanded for 

 the establishing of a zonal determination, at the same time it is 

 urged that the conclusions advanced are the result of the examina- 

 tion of many hundreds of zonally-collected examples, and that they 

 do give a fair average reading of the special zonal characters. With 

 so Protean a genus as this, there can be no hard-and-fast rules, and 

 there may be trivial exceptions, which in no way weaken the general 

 conclusions. Take an instance from the interporiferous area, for 

 example : so gentle is the transition from one form to another, 

 that in the same specimen we may sometimes see two types of 

 areas — three ambulacra being ' divided,' and one ' subdivided,' and 

 so on. But, after all, this is how Nature works. 



